In the turbine stages of a gas turbine engine there are considerable efficiency losses associated with the aerodynamics of the trailing edges of the aerofoils. It is known that these losses can be greatly reduced if the trailing edge can be made consistently with a thinner profile, which improves the turbine efficiency and therefore decreases the specific fuel consumption.
Turbine aerofoil components are conventionally manufactured by an investment casting process. This process inherently has limits on how thin sections can be cast due to surface tension effects. Typically, the smallest trailing edge thickness which can be reliably cast using the process is around 0.6 mm. To reliably manufacture trailing edge thicknesses of less than 0.6 mm it is desirable to carry out an operation such as machining of the cast component. To preserve the aerodynamic properties of the convex surface of the aerofoil it is further desirable that the material is removed from the concave surface during the thinning operation.
It is known to use fully adaptive machining processes to thin the trailing edge of the cast aerofoil. One example of such an adaptive machining process is disclosed US patent application publication no. US2014/373503A. Such processes are very effective, but very expensive adding significantly to unit cost of manufacture.
Fixtures for holding aerofoils during machining processes are known. For example, a 6 point location nest is described in US patent application publication no. US2014/191455A. Whilst such arrangements can secure a part, they do not align a trailing edge for accurate machining with respect to the suction surface of the aerofoil. Also, these known arrangements do not provide unencumbered access to a machine tool along the entire length of the trailing edge.
Another example of a known fixture arrangement is described in the applicant's own US patent application publication US2013318773A. Whilst the described arrangement allows good access to the trailing edge, the arrangement does not provide for accurate alignment of the trailing edge for machining by a conventional machine tool, an accurate result would require an adaptive machining solution.
The invention seeks to provide a fixture which sets a cast aerofoil in an orientation which is suitable for machining by a direct machining process such as milling.